Endpoint metadata ADT and specs

[sergeykolbasov]

Related to #581

@vkostyukov @rpless please, take a look. It's the first iteration and I'd be glad to get some feedback on this.

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@9f677b3). Click here to learn what that means.
The diff coverage is 44.82%.

@@            Coverage Diff            @@
##             master     #880   +/-   ##
=========================================
  Coverage          ?   79.47%           
=========================================
  Files             ?       52           
  Lines             ?      794           
  Branches          ?       35           
=========================================
  Hits              ?      631           
  Misses            ?      163           
  Partials          ?        0

Impacted Files	Coverage Δ
core/src/main/scala/io/finch/endpoint/path.scala	76.31% <0%> (ø)
...tee/src/main/scala/io/finch/iteratee/package.scala	91.3% <0%> (ø)
core/src/main/scala/io/finch/endpoint/cookie.scala	50% <100%> (ø)
...rc/main/scala/io/finch/syntax/EndpointMapper.scala	100% <100%> (ø)
core/src/main/scala/io/finch/endpoint/header.scala	64.7% <100%> (ø)
...e/src/main/scala/io/finch/endpoint/multipart.scala	77.41% <100%> (ø)
core/src/main/scala/io/finch/endpoint/param.scala	79.41% <100%> (ø)
core/src/main/scala/io/finch/Endpoint.scala	67.83% <28.57%> (ø)
core/src/main/scala/io/finch/endpoint/body.scala	58.33% <50%> (ø)
core/src/main/scala/io/finch/Endpoints.scala	66.66% <66.66%> (ø)

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